Densely patterned contact holes form a key component of integrated circuits, but there are significant challenges to patterning holes with a sub 10-nm radius. The directed self-assembly (DSA) of block copolymers offers a potential solution, where a larger hole is initially patterned and a cylinder forming block copolymer (BCP) is assembled inside. The radius of the inner BCP cylinder can now be used to template the hole radius. It can be particularly challenging to characterize the internal structure of the DSA patterned contact hole, particularly for features such as the residual layer, which may adversely impact the etching process. The high aspect ratio of these features makes top-down characterization nearly impossible, forcing the use of challenging cross-section based approaches.
Critical Dimension small angle X-ray scattering (CDSAXS) offers the possibility of characterizing the internal structure of DSA patterned contact holes. CDSAXS is a variable angle scattering technique which combines measurements from different sample angles to reconstruct the three-dimensional structure of the sample. In this case the scattering yields a two-dimensional pattern, where the off-axis peaks contain additional information about the structure. A model based inverse analysis is then used to fit the scattering and evaluate the structure of the measured target. Contact holes were prepared with a variety of radii and surface treatments in order to evaluate which conditions resulted in optimal assembly of the BCP inside the template. The results demonstrate a correlation between hole radius, surface treatment and residual layer thickness.

With the advent of high brightness sources and fast detectors, there is a possibility for combining fast X-ray acquisition with high-speed data treatment to reach the timescale for an effective in-line characterization method. We will highlight two recent developments using Small Angle X-ray Scattering on nanoscale etched patterns: the first is the inclusion of a CD-SAXS tool, allowing the data treatment and simulations to reconstruct the form-factor, inside the Xi-cam framework; the second is the development of a high performance Grazing Incidence approach to reconstruct the shape of line profile. This study also shows the comparison between the line profiles reconstructed from both techniques as well as the profile extracted from cross-section SEM.

The semiconductor industry is evaluating a variety of approaches for the cost efficient production of future processing and memory generations. Amongst the technologies being explored are multiple patterning steps, extreme ultraviolet (EUV) lithography, multiple-beam electron beam lithography and the directed self-assembly (DSA) of block copolymers (BCPs). BCP DSA utilizes a chemical or topographical template to induce long range order in a thin film of BCP which enhances the resolution of the original pattern.
The characterization of buried structure within a DSA BCP film is challenging due to the lack of contrast between the organic materials. Critical-dimension small angle x-ray scattering (CDSAXS) measurements were performed on DSA BCP films, using soft X-rays to tune the contrast, in order to understand the relationship between template structure and film morphology.1 The results of these measurements show that as the width of the guiding stripe widens the arrangement of the BCP on the guiding stripe inverts, shifting from the A block being centered on the guiding stripe to the B block being centered on the guiding stripe. The initial results of integration of mean field simulations into the analysis of scattering data will also be discussed.
In addition to examining the BCP structure with CDSAXS, soft X-ray reflectivity2 measurements were performed on BCP to better understand the relationship between interface width for systems with alternative architectures (triblocks) and additives (polymers/ionic liquids). The addition of a selectively associating additive increases the interaction parameter between the two blocks, resulting in the reduction of the interface width and access to smaller pitches. The use of soft X-ray reflectivity allows the evaluation of the distribution of the additive.
(1) Sunday, D. F.; Hammond, M. R.; Wang, C.; Wu, W.; Delongchamp, D. M.; Tjio, M.; Cheng, J. Y.; Kline, R. J.; Pitera, J. W. Determination of the Internal Morphology of Nanostructures Patterned by Directed Self Assembly. ACS Nano 2014, 8, 8426–8437.
(2) Sunday, D. F.; Kline, R. J. Reducing Block Copolymer Interfacial Widths through Polymer Additives. Macromolecules 2015, 48, 679–686.

New critical dimension metrology methods such as critical dimension small angle X-ray scattering (CDSAXS) are being developed to meet the measurement challenges of next generation devices. Two key requirements for any new CD metrology method are non-destructiveness and the measurement speed. We will report on a study of beam damage and scattering strength of two model photoresist systems, HSQ and PMMA. We also will report on the status and initial results from NIST’s upgraded lab CDSAXS system.
50 nm pitch line gratings were fabricated in HSQ and PMMA films using EUV interference lithography at the Swiss Light Source. The lines were about 30 nm tall and 20-30 nm wide. The 17 keV CDSAXS exposure time was varied from 0.1 s to 60 s to determine the minimum X-ray exposure required to obtain a satisfactory fit. Normal incident measurements separated by a blanket X-ray exposure were repeated to measure the decrease in scattering intensity with X-ray dose. The PMMA scattering signal was found to decrease by about 80 % before stabilizing at around 15 % of the original scattering intensity. The HSQ scattering signal decreased much less and stabilized at about 80 % of the original scattering intensity. We also conducted a series of variable-angle CDSAXS measurements as a function of blanket X-ray exposure to determine how the shape of the photoresist lines changed during X-ray exposure. For PMMA, we found the line width to remain constant and the line height to decrease from 25 nm to 10 nm during the exposure series. The exposures that damaged the samples corresponded to several hours of exposure to the synchrotron beam in a 100 µm spot and were much longer than what was required to characterize the line gratings. Smaller targets result in a larger dose and could potentially damage the resist in the time required to make a CDSAXS measurement. The large differences in beam damage between PMMA and HSQ show that resist damage from CDSAXS will depend on the particular resist chemistries and target size.

Semiconductor devices continue to shrink in size with every generation. These ever smaller structures are challenging the resolution limits of current analytical and inline metrology tools. We will discuss the results of a study of critical dimension small angle x-ray scattering (CDSAXS) comparing the measured intensity from a laboratory source and a synchrotron to determine the improvements in compact x-ray source technology necessary to make CDSAXS a high throughput metrology method. We investigated finFET test structures with and without a high-k gate dielectric coating. The HfO2-based high-k gate dielectric substantially increased the scattering intensity. We found that single-angle laboratory source measurements of 15 min from HfO2-coated finFETs had sufficient scattering intensity to measure the higher order peaks necessary for obtaining high-resolution dimensional fits. Identical bare silicon finFETs required at least 2 h of exposure time for equivalent data quality. Using these structures, we measured the scattering efficiency and determined the required photon flux for next generation x-ray sources to make an inline CDSAXS tool high throughput.

We compare the speed and effectiveness of two genetic optimization algorithms to the results of statistical sampling via a Markov chain Monte Carlo algorithm to find which is the most robust method for determining real-space structure in periodic gratings measured using critical dimension small-angle x-ray scattering. Both a covariance matrix adaptation evolutionary strategy and differential evolution algorithm are implemented and compared using various objective functions. The algorithms and objective functions are used to minimize differences between diffraction simulations and measured diffraction data. These simulations are parameterized with an electron density model known to roughly correspond to the real-space structure of our nanogratings. The study shows that for x-ray scattering data, the covariance matrix adaptation coupled with a mean-absolute error log objective function is the most efficient combination of algorithm and goodness of fit criterion for finding structures with little foreknowledge about the underlying fine scale structure features of the nanograting.

A line grating prepared via a self-aligned quadruple patterning method was measured using critical dimension small angle x-ray scattering. A Monte Carlo Markov chain algorithm was used to analyze the uncertainty of the model fit over subsets of the full angular range and for a time series with decreasing signal-to-noise in order to determine the effect of the data quality on the final profile shape uncertainty. These results show how the total measurement time can be reduced while maintaining satisfactory profile shape uncertainty. We found that the typical measurement conditions are highly oversampled and can be reduced considerably with only marginal effect on the shape uncertainty. A comparison is made between the synchrotron measurements and a laboratory system, demonstrating that both measurements result in similar structures.

There has been significant interest in hybrid metrology as a novel method for reducing overall measurement uncertainty and optimizing measurement throughput (speed) through rigorous combinations of two or more different measurement techniques into a single result. This approach is essential for advanced 3-D metrology when performing model-based critical dimension measurements. However, a number of fundamental challenges present themselves with regard to consistent noise and measurement uncertainty models across hardware platforms, and the need for a standardized set of model parameters. This is of paramount concern when the various techniques have substantially different models and underlying physics. In this paper we present realistic examples using scanning electron microscopy, atomic force microscopy, and optical critical dimension (CD) methods applied to sub-20 nm dense feature sets. We will show reduced measurement uncertainties using hybrid metrology on 15 nm CD features and evaluate approaches to adapt quantitative hybrid metrology into a high volume manufacturing environment.

We report on the development of a new measurement method, resonant critical-dimension small-angle x-ray scattering (res-CDSAXS), for the characterization of the buried structure of block copolymers (BCP) used in directed self assembly (DSA). We use resonant scattering at the carbon edge to enhance the contrast between the two polymer blocks and allow the determination of the three-dimensional shape of the native lamella in a line–space pattern by CDSAXS. We demonstrate the method by comparing the results from conventional CDSAXS to res-CDSAXS on a 1:1 DSA BCP sample with a nominal 50-nm pitch. The res-CDSAXS method provides substantially improved uncertainty in the fit of the line shape and allows the determination of the buried structure.

In this paper, we present a comparison of profile measurements of vertical field effect transistor (FinFET) fin arrays by optical critical dimension (OCD) metrology and critical dimension small angle X-ray scattering (CD-SAXS) metrology. Spectroscopic Muller matrix elements measurements were performed at various azimuthal angles for OCD, and X-ray diffraction intensities were collected for different incident angles in CD-SAXS measurements. A common trapezoidal model was used to compute the OCD and CD-SAXS signatures, using rigorous coupled wave (RCW) analysis and a 2D Fourier transform, respectively. Profile parameters, some material parameters, and instruments parameters were adjusted by a non-linear fitting procedure of the data. Results from both measurement techniques were compared and found in reasonable agreement with one another, although some of the parameters have differences that exceed the estimated uncertainties.

We have demonstrated that transmission critical dimension small angle X-ray scattering (CD-SAXS) provides high accuracy and precision CD measurements on advanced 3D microelectronic architectures. The competitive advantage of CD-SAXS over current 3D metrology methods such as optical scatterometry is that CD-SAXS is able to decouple and fit cross-section parameters without any significant parameter cross-correlations. As the industry aggressively scales beyond the 22 nm node, CD-SAXS can be used to quantitatively measure nanoscale deviations in the average crosssections of FinFETs and high-aspect ratio (HAR) memory devices. Fitting the average cross-section of 18:1 isolated HAR contact holes with an effective trapezoid model yielded an average pitch of 796.9 ± 0.4 nm, top diameter of 70.3 ± 0.9 nm, height of 1088 ± 4 nm, and sidewall angle below 0.1°. Simulations of dense 40:1 HAR contact holes and FinFET fin-gate crossbar structures have been analyzed using CD-SAXS to inquire the theoretical precision of the technique to measure important process parameters such as fin CD, height, and sidewall angle; BOX etch recess, thickness of hafnium oxide and titanium nitride layers; gate CD, height, and sidewall angle; and hafnium oxide and titanium nitride etch recess. The simulations of HAR and FinFET structures mimic the characteristics of experimental data collected at a synchrotron x-ray source. Using the CD-SAXS simulator, we estimate the measurement capabilities for smaller similar structures expected at future nodes to predict the applicability of this technique to fulfill important CD metrology needs.

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